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The following remarks are found in the end chapter notes to Chapter 22, Elementary Graph Algorithms of Corman, et al., Introduction to Algorithms, 2nd. ed., MIT Press, 2001, on page 560:

Breadth-first search was discovered by Moore [226] in the context of finding paths through mazes. Lee [198] independently discovered the same algorithm in the context of routing wires on circuit boards.

The relevant references are:

E. F. Moore (1959), The shortest path through a maze. In Proceedings of the International Symposium on the Theory of Switching, Harvard University Press, pp. 285–292.